CHENNAI, India (CNN) -- As dawn broke Monday across the Bay of Bengal, countries struck by tsunamis in the wake of the most powerful earthquake the planet has seen in 40 years focused on relief and rescue efforts, and said the death toll from the giant waves -- already more than 13,000 -- is expected to rise further.

The tsunamis also left thousands injured, thousands missing and hundreds of thousands homeless in Indonesia, India, Sri Lanka and Thailand.

Some of the tsunamis reached as far as 1,600 kilometers (1,000 miles) from the epicenter of the 9.0 magnitude quake, which was located about 160 kilometers (100 miles) off the coast of Indonesia's Sumatra Island at a depth of about 10 kilometers (6.2 miles).

The quake struck about 7 a.m. Sunday (midnight GMT Saturday), according to the U.S. Geological Survey's National Earthquake Center.

It is the fourth-largest earthquake since such measurements began in 1899, according to the NEIC, tying with a 1952 quake in Kamchatka, Russia.

More than 4,500 people have been reported dead in Sri Lanka. Most of them, authorities said, were in the eastern district of Batticaloa. Thousands were missing and more than a half million displaced.

The Sri Lankan government declared a state of emergency, and, along with the government of the Maldives, has requested international assistance, the United Nations' Office for the Coordination of Humanitarian Affairs reported.

The United Nations has warned of epidemics within days unless health systems in the affected areas can cope.

"This may be the worst natural disaster in recent history because it is affecting so many heavily populated coastal areas ... so many vulnerable communities," the U.N.'s Emergency Relief Coordinator Jan Egeland told CNN. (Full story)

As the sun rose, 20,000 Sri Lankan soldiers and naval personnel launched relief and rescue efforts. India sent six warships, carrying supplies, along with helicopters.

Priorities including identifying the hardest-hit areas and air-dropping supplies, along with shepherding stranded people to safer areas.

Sri Lankan authorities imposed a curfew overnight, and many residents remained concerned about the possibility of additional tsunamis.

The country has been in the throes of a civil war, and land mines uprooted by the waves were hampering relief efforts.

Some tourists, meanwhile, had been evacuated from the hard-hit eastern coasts to the capital, Colombo, on the west coast and unaffected.

At first light, many Sri Lankans ventured out to scour the debris for belongings or to search for information on missing family members.

Although India was giving aid to Sri Lanka, that country also was reeling from the aftermath of the quake and tsunamis. India's official government news agency, Press Trust of India, said at least 4,000 Indians were killed, and more bodies were being recovered.

A resident of Chennai (formerly Madras) in Tamil Nadu district -- India's hardest-hit area -- said he saw several people being swept out to sea.

Along India's southeastern coast, several villages appeared to have been swept away, and thousands of fishermen -- including 2,000 from the Chennai area alone -- who were at sea when the waves thundered ashore have not returned. (Full story)

Along the coast, brick foundations were all that remained of village homes. In Tamil Nadu state, 1,725 people have been confirmed dead, and officials feared many more died on the remote Andaman and Nicobar islands, where dozens of aftershocks were centered, but communication with the mainland was cut off.

Efforts to provide survivors with food and shelter were hampered by the overwhelming magnitude of the damage.

Thai authorities said more than 400 people are dead, and hundreds are missing.

One witness said Phuket's famed Laguna Beach resort area is "completely gone." The area provided 40 percent of Thailand's $10 billion annual income from tourism.

Among the missing were scuba divers who had been exploring the Emerald Cave off Phuket's coast.

Phuket's airport -- which closed when its runways flooded -- reopened, but most roads in the area remained closed, as officials tried to assess the damage.

Others reported narrow escapes -- including a Spaniard who had been aboard a boat when the wave approached.

The captain began screaming and turned the boat directly into a nearby shore, where he beached it.

As those aboard jumped from the craft and scrambled up the steep beach, they turned back to see the waves crush their boat, the Spaniard said.

Communication difficulties

More than 500 people have been confirmed dead in Indonesia -- many of them in Aceh, in northern Sumatra, about 160 kilometers (100 miles) from the quake's epicenter, according to local reports.

The quake also inflicted heavy damage on the area, which is a hotbed of rebel activity, before two tsunamis slammed the coastline.

Access and communications were difficult if not impossible; the death toll remained a mystery on the west coast of Aceh, where communications had been completely wiped out. News agencies in the country have reported more than 4,000 dead.

The tsunamis struck with no warning to those in coastal areas, as no warning system exists for the Indian Ocean, said Eddie Bernard, director of the National Oceanic and Atmospheric Administration's Pacific Marine and Environmental Labs in Seattle.

Staffers at warning centers that cover the Pacific Basin and the U.S. West Coast were aware of the quake and the possibility of tsunamis, said Laura Kong, director of the International Tsunami Information Center in Honolulu, Hawaii.

"They were able to make contact, but they did not have the proper government officials to notify," she said. "They'll be working on this in the future."

The earthquake is classified as "great" -- the strongest classification given by the NEIC.

NEIC geophysicist Don Blakeman said the tsunamis were triggered by the initial massive jolt.

"The damage is just phenomenal," said Jan Egelund, U.N. emergency relief coordinator. "I think we are seeing now one of the worst natural disasters ever."

Aftershocks

There was disagreement over whether the threat was over. Waverly Person, Blakeman's colleague at NEIC, said the tsunamis are "long over" and residents and visitors should not worry about further tsunamis.

Bernard, however, said the aftershocks are strong enough to produce more tsunamis.

One such aftershock, measuring 7.3 in magnitude, struck about 300 kilometers (200 miles) northwest of Banda Aceh -- on Sumatra's northernmost tip -- more than four hours after the initial quake, according to the NEIC.

The center expects the quake to produce hundreds of smaller aftershocks, under 4.6 magnitude, and thousands smaller than that.

"A quake of this size has some pretty serious effects," Person said.

The quake represented the energy released from "a very large rupture in the earth's crust" more than 1,000 kilometers (600 miles) long. The rupture created shock waves that pushed the water at speeds of up to several hundred kilometers per hour.

It was the strongest earthquake to hit anywhere on Earth since March 1964, when a 9.2 quake struck near Alaska's Prince William Sound.

The strongest recorded earthquake (and records go back to 1899) registered 9.5 on May 22, 1960, in Chile.

Sunday's quake hit a year after the 6.6-magnitude quake in Bam, Iran, which killed more than 30,000 people, injured another 30,000 and destroyed 85 percent of the buildings in the southeastern Iran city.